This invention relates generally to differential amplifiers having a single-ended voltage input and a differential current output, and more specifically to a method and apparatus for improving the balance of the differential output. A prior art amplifier 10 for converting a single-ended voltage into a differential current is shown in FIG. 1. This amplifier includes a differential stage (Q1, Q2), a single-ended voltage input 12 for receiving a single-ended voltage vi, a differential current output 14, and a bias terminal 16 for receiving a bias tail current. Load resistors RL coupled to the positive power supply Vdd are provided for converting the differential current output 14 into a differential voltage (vo1, vo2). A bias circuit such as resistor R1 coupled to the negative supply voltage Vss supplies the bias tail current to the bias terminal 16 of the differential stage. Alternatively, R1 may model the output resistance of a current source circuit that provides the bias tail current to the differential amplifier. The incremental voltage at bias terminal 16 is designated v1 and is typically equal to one half of the input voltage vi at signal frequencies. However, since resistor R1 has a finite output impedance, an associated undesirable signal tail current equal to v1/R1 flows out of the differential stage (Q1, Q2).
The effect of the signal tail current is shown in FIG. 1A. The single-ended voltage vi is shown as a sine wave. The two single-ended components of the differential output voltage vo1 and vo2 are shown as inverted and noninverted voltage outputs, respectively. In an ideal amplifier, the drain currents id1 and id2 that constitute the single-ended components of the differential current output 14 are equal. Therefore, the magnitudes of voltage components vo1 and vo2 are equal. However, the signal tail current adds to drain current id1 and subtracts from id2, as shown in the corresponding vo1 and vo2 waveforms, making the magnitudes unequal.
The unbalanced differential output is particularly noticeable in amplifiers constructed with short channel FET transistors such as GaAs FETs (field effect transistors). In a GaAs differential amplifier it is difficult to implement a high impedance bias tail current, particularly when using small power supplies. In many cases, biasing resistors are used instead as shown in FIG. 1 since the associated voltage drop is low. In this case the undesirable signal tail current increases as the value of the biasing resistor decreases.
The problem caused by the unbalanced differential output in the prior art amplifiers is that signal quality in subsequent signal processing stages may be adversely affected. For example, as one of the output voltage components diminishes, subsequent logic stages may fail to switch. Conversely, as the other of the output voltage components increases, subsequent amplifier stages may saturate, causing distortion. Many RF circuits such as mixers require highly balanced differential signals for proper operation. The imbalance in the output typically increases with frequency because of the parasitic capacitance at bias terminal 16, resulting in even greater signal degradation.
What is desired is a compensation method and apparatus for single-ended to differential amplifiers that improves the balance in the differential output.